Groundwater and Surface Water

A Single Resource in Washington

September 1, 2010

The need for coordinating the management and use of groundwater and surface water is becoming recognized throughout the western United States and much of the world. Although groundwater is a relatively small portion of the overall hydrologic cycle it is a critically important component in the natural movement of water through most watersheds. In part because groundwater was not well understood, its regulation and management has historically been approached separately and distinctly from surface water. However, as the scientific understanding of groundwater has increased, it has become apparent that groundwater and surface water use can only be effectively managed as a single resource.

All groundwater withdrawals result in impacts to the hydrologic system in which they occur. Although most individual withdrawals are relatively small and frequently undetectable at the basin scale, each new withdrawal adds to the total being consumed. And in the last 60 years, throughout most of Washington, the amount of groundwater withdrawn and consumed has increased each year.

In most cases, the hydrologic impacts from pumping manifest themselves as reductions in storage (declining water levels in the aquifer over time), reduced discharge (decrease in the amount of water flowing out of the aquifer), and increased recharge (increase in the amount of water flowing into the aquifer). Because rivers and streams are frequently the primary natural drain for most aquifers tapped by wells, pumping and consuming groundwater almost always impacts surface water bodies. Most of the rivers and streams being impacted by pumping do not have water available without negative consequences to existing instream resources or impairment of senior water rights.

Late summer streamflow in most of Washington’s rivers and streams is dependent on groundwater draining into the streambed. During the drier summer months when flows are typically the lowest of the year, groundwater flowing into the stream is frequently providing almost all of the streamflow. Groundwater also provides a source of cooler water which is critical to fish reproduction and survival. It is widely known and accepted by hydrogeologists that use and consumption of groundwater typically results in decreases in streamflow (by a decrease in groundwater flow into the stream or an increase in surface water leaking out of a stream). There are literally thousands of published references describing and quantifying groundwater – surface water interactions and why this occurs, including many good reports regarding conditions throughout Washington.

It is also becoming widely apparent through legal opinions and court rulings that surface and groundwater must be managed together and that groundwater withdrawals can and do impair surface water rights. This is consistent with the recommendation from Robert Hirsch, the former Chief Hydrologist for the USGS, who wrote “Effective policies and management practices must be built on a foundation that recognizes that surface water and ground water are simply two manifestations of a single integrated resource.” See: http://pubs.usgs.gov/circ/circ1139/#pdf Additional selected quotes and figures from USGS Circular 1139 are included in Attachment 1.

Another particularly relevant publication regarding the wise and sustainable use of groundwater as well as the impacts from development is USGS Circular 1186. See: http://pubs.usgs.gov/circ/circ1186/pdf/circ1186.pdf Selected quotes and figures from USGS Circular 1186 are included as Attachment 2.

ATTACHMENT 1

USGS Circular 1139 – Ground Water and Surface Water A Single Resource

Selected Figures and Quotes

“Surface water commonly is hydraulically connected to ground water, but the interactions are difficult to

observe and measure”

“Ground water moves along flow paths of varying lengths in transmitting water from areas of recharge

to areas of discharge”

Figure 3. Ground-water flow paths vary greatly in length, depth, and traveltime from points of recharge

to points of discharge in the groundwater system.

Figure A-2. Using known altitudes of the water

table at individual wells (A), contour maps of the

water-table surface can be drawn (B), and

directions of ground-water flow along the water

table can be determined (C) because flow usually is

approximately perpendicular to the contours.

Figure 8. Gaining streams receive water from

the ground-water system (A). This can be

determined from water-table contour maps

because the contour lines point in the upstream

direction where they cross the stream (B).

Figure 9. Losing streams lose water to the

ground-water system (A). This can be

determined from water-table contour maps

because the contour lines point in the

downstream direction where they cross the

stream (B).

Figure 10. Disconnected streams are separated

from the ground-water system by an

unsaturated zone.

Figure 11. If stream levels rise higher than

adjacent ground-water levels, stream water

moves into the streambanks as bank storage.

“Streams interact with ground water in three basic ways: streams gain water from inflow of ground

water through the streambed (gaining stream), they lose water to ground water by outflow through the

streambed (losing stream), or they do both, gaining in some reaches and losing in other reaches”

Ground water contributes to streams in most physiographic and climatic settings. The amount of water that ground water contributes to streams can be estimated by analyzing streamflow hydrographs to determine the ground-water component, which is termed base flow (Figure B–1).

Figure B–1. The ground-water component of streamflow was estimated from a streamflow hydrograph

for the Homochitto River in Mississippi, using a method developed by the institute of Hydrology, United

Kingdom. (Institute of Hydrology, 1980, Low flow studies: Wallingford, Oxon, United Kingdom, Research

Report No. 1.)

Figure B–2. In the conterminous United States, 24 regions were delineated where the interactions of ground water and surface water are considered to have similar characteristics. The estimated ground-water contribution to streamflow is shown for specific streams in 10 of the regions.

The Effect of Ground-Water Withdrawals on Surface Water Withdrawing water from shallow aquifers that are directly connected to surface-water bodies can have a significant effect on the movement of water between these two water bodies. The effects of pumping a

single well or a small group of wells on the hydrologic regime are local in scale. However, the effects of

many wells withdrawing water from an aquifer over large areas may be regional in scale. In the long term, the quantity of ground water withdrawn is approximately equal to the reduction in streamflow that is potentially available to downstream users.

Figure C–1. In a schematic hydrologic setting where ground water discharges to a stream under natural conditions (A), placement of a well pumping at a rate (Q1) near the stream will intercept part of the ground water that would have discharged to the stream (B). If the well is pumped at an even greater rate (Q2), it can intercept additional water that would have discharged to the stream in the vicinity of the well and can draw water from the stream to the well (C).

Interaction of Ground Water and Surface Water in Mountainous Terrain

Figure 20. Water from precipitation moves to mountain streams along several pathways. Between storms and snowmelt periods, most inflow to streams commonly is from ground water (A). During storms and snowmelt periods, much of the water inflow to streams is from shallow flow in saturated macropores in the soil zone. If infiltration to the water table is large enough, the water table will rise to the land surface and flow to the stream is from ground water, soil water, and overland runoff (B). In arid areas where soils are very dry and plants are sparse, infiltration is impeded and runoff from precipitation can occur as overland flow (C).

Figure 21. In mountainous terrain, ground water can discharge at the base of steep slopes (left side of valley), at the edges of flood plains (right side of valley), and to the stream.

Interaction of Ground Water and Surface Water in Riverine Terrain

Figure 22. In broad river valleys, small local ground-water flow systems associated with terraces overlie more regional ground-water flow systems. Recharge from flood waters superimposed on these ground-water flow systems further complicates the hydrology of river.

ATTACHMENT 2

USGS Circular 1186 – Sustainability of Ground-Water Resources

Selected Figures and Quotes

“The sustainability of ground-water resources is a function of many factors, including depletion of

ground-water storage, reductions in streamflow, potential loss of wetland and riparian ecosystems, land

subsidence, saltwater intrusion, and changes in ground-water quality.” – Charles G. Groat, Director,

USGS.

Ground water is one of the Nation’s most important natural resources. Ground water is a major contributor to flow in many streams and rivers and has a strong influence on river and wetland habitats for plants and animals. From an overall national perspective, the ground-water resource appears ample. Locally, however, the availability of ground water varies widely. Moreover, only a part of the ground water stored in the subsurface can be recovered by wells in an economic manner and without adverse consequences. Ground water is not a nonrenewable resource, such as a mineral or petroleum deposit, nor is it completely renewable in the same manner and timeframe as solar energy. If sustainable development is to mean anything, such development must be based on an appropriate understanding of the environment—an environment where knowledge of water resources is basic to virtually all endeavors.

Figure 4. The unsaturated zone, capillary fringe, water table, and saturated zone.

GENERAL FACTS AND CONCEPTS ABOUT GROUND WATER

Ground water occurs almost everywhere beneath the land surface.

Natural sources of freshwater that become ground water are (1) areal recharge from precipitation that percolates through the unsaturated zone to the water table (Figure 4) and (2) losses of water from streams and other bodies of surface water such as lakes and wetlands.

The top of the subsurface ground-water body, the water table, is a surface, generally below the land surface, that fluctuates seasonally and from year to year in response to changes in recharge from precipitation and surface water bodies.

Ground water commonly is an important source of surface water.

Ground water serves as a large subsurface water reservoir.

Velocities of ground-water flow generally are low and are orders of magnitude less than velocities of streamflow.

Under natural conditions, ground water moves along flow paths from areas of recharge to areas of discharge at springs or along streams, lakes, and wetlands.

The areal extent of ground-water-flow systems varies from a few square miles or less to tens of thousands of square miles.

The age (time since recharge) of ground water varies in different parts of ground-water flow systems.

Surface and subsurface earth materials are highly variable.

Earth materials vary widely in their ability to transmit and store ground water.

Wells are the principal direct window to study the subsurface environment.

Pumping ground water from a well always causes (1) a decline in ground-water levels (heads; see Figure 7) at and near the well, and (2) a diversion to the pumping well of ground water that was moving slowly to its natural, possibly distant, area of discharge.

Ground-water heads respond to pumping to markedly different degrees in unconfined and confined aquifers.

Figure 5. A local scale ground-water-flow system.

In this local scale ground-water-flow system, inflow of water from areal recharge occurs at the water table. Outflow of water occurs as (1) discharge to the atmosphere as ground-water evapotranspiration (transpiration by vegetation rooted at or near the water table or direct evaporation from the water table when it is at or close to the land surface) and (2) discharge of ground water directly through the streambed. Short, shallow flow paths originate at the water table near the stream. As distance from the stream increases, flow paths to the stream are longer and deeper. For long-term average conditions, inflow to this natural ground-water system must equal outflow.

Figure 6. A regional ground-water-flow system that comprises subsystems at different scales and a complex hydrogeologic framework. Significant features of this depiction of part of a regional ground-water-flow system include (1) local ground-water subsystems in the upper water-table aquifer that discharge to the nearest surface-water bodies (lakes or streams) and are separated by ground-water divides beneath topographically high areas; (2) a subregional ground-water subsystem in the water-table aquifer in which flow paths originating at the water table do not discharge into the nearest surface-water body but into a more distant one; and (3) a deep, regional ground-water-flow subsystem that lies beneath the water-table subsystems and is hydraulically connected to them. The hydrogeologic framework of the flow system exhibits a complicated spatial arrangement of high hydraulic-conductivity aquifer units and low hydraulic-conductivity confining units. The horizontal scale of the figure could range from tens to hundreds of miles.

GROUND-WATER DEVELOPMENT, SUSTAINABILITY, AND WATER BUDGETS The one common factor for all ground-water systems, however, is that the total amount of water entering, leaving, and being stored in the system must be conserved. An accounting of all the inflows, outflows, and changes in storage is called a water budget. Human activities, such as ground-water withdrawals and irrigation, change the natural flow patterns, and these changes must be accounted for in the calculation of the water budget. Because any water that is used must come from somewhere, human activities affect the amount and rate of movement of water in the system, entering the system, and leaving the system. Some hydrologists believe that a predevelopment water budget for a ground-water system (that is, a water budget for the natural conditions before humans used the water) can be used to calculate the amount of water available for consumption (or the safe yield). In this case, the development of a ground-water system is considered to be “safe” if the rate of ground-water withdrawal does not exceed the rate of natural recharge. This concept has been referred to as the “Water-Budget Myth” (Bredehoeft and others, 1982). It is a myth because it is an oversimplification of the information that is needed to understand the effects of developing a ground-water system. As human activities change the system, the components of the water budget (inflows, outflows, and changes in storage) also will change and must be accounted for in any management decision. Understanding water budgets and how they change in response to human activities is an important aspect of ground-water hydrology; however, as we shall see, a predevelopment water budget by itself is of limited value in determining the amount of ground water that can be withdrawn on a sustained basis.

Figure 8. Diagrams illustrating water budgets for a ground-water system for predevelopment and development conditions. (A) Predevelopment water-budget diagram illustrating that inflow equals outflow. (B) Water-budget diagram showing changes in flow for a ground-water system being pumped. The sources of water for the pumpage are changes in recharge, discharge, and the amount of water stored. The initial predevelopment values do not directly enter the budget calculation.

The source of water for pumpage is supplied by (1) more water entering the ground-water system (increased recharge), (2) less water leaving the system (decreased discharge), (3) removal of water that was stored in the system, or some combination of these three. Because any use of ground water changes the subsurface and surface environment (that is, the water must come from somewhere), the public should determine the tradeoff between ground-water use and changes to the environment and set a threshold for what level of change becomes undesirable. As development of land and water resources intensifies, it is increasingly apparent that development of either ground water or surface water affects the other. From a sustainability perspective, the key point is that pumping decisions today will affect surface-water availability; however, these effects may not be fully realized for many years.

Figure 14. The principal source of water to a well can change with time from ground-water storage to capture of streamflow. The percentage of ground-water pumpage derived from ground-water storage and capture of streamflow (decrease in ground-water discharge to the stream or increase in ground-water recharge from the stream) is shown as a function of time for the hypothetical stream-aquifer system shown in Figure 13. A constant pumping rate of the well is assumed. For this simple system, water derived from storage plus streamflow capture must equal 100 percent. The time scale of the curves shown depends on the hydraulic characteristics of the aquifer and the distance of the well from the stream. A key feature of some aquifers and ground-water systems is the large volume of ground water in storage, which allows the possibility of using aquifers for temporary storage, that is, managing inflow and outflow of ground water in storage in a manner similar to surface-water reservoirs. The foundation of any good ground-water analysis, including those analyses whose objective is to propose and evaluate alternative management strategies, is the availability of high-quality data.

Strategies for Sustainability

Use sources of water other than local ground water.

Change rates or spatial patterns of ground-water pumpage.

Increase recharge to the ground-water system.

Decrease discharge from the ground-water system.

Change the volume of ground water in storage at different time scales.

Innovative approaches that have been undertaken to enhance the sustainability of ground-water resources typically involve some combination of use of aquifers as storage reservoirs, conjunctive use of surface water and ground water, artificial recharge of water through wells or surface spreading, and the use of recycled or reclaimed water.

Concluding Remarks

The most important and most extensively discussed concept in this report is that volumes of water pumped from a groundwater system must come from somewhere and must cause a change in the groundwater system. Possible sources of water for pumpage are (1) more water entering the ground-water system (increased recharge), (2) less water leaving the system (decreased discharge), and (3) removal of water that was stored in the system.

One of the critical linkages in both unstressed and stressed ground-water systems is between ground water and surface water.

Continuing large withdrawals of water from an aquifer often result in undesirable consequences. From a management standpoint, water managers, stakeholders, and the public must decide the specific conditions under which the undesirable consequences can no longer be tolerated.

The effects of ground-water development may require many years to become evident. Thus, there is an unfortunate tendency to forego the data collection and analysis that is needed to support informed decision making until well after problems materialize.